5 1.2 DNA:Form and Function Figure 1.5 The double helix.The double-helical structure of DNA proposed by Watson and Crick The sugar-phosphate backbones of the two chains are shown in red and blue,and the bases are shown in green,purple,orange,and yellow.The two strands are antiparallel,running in opposite directions with respect to the axis of the double helix,as indicated by the arrows. Two single strands of DNA combine to form a double helix Most DNA molecules consist of not one but two strands(Figure 1.5).In 1953,James Watson and Francis Crick deduced the arrangement of these strands and proposed a three-dimensional structure for DNA molecules. This structure is a double helix composed of two intertwined strands arranged such that the sugar-phosphate backbone lies on the outside and the bases on the inside.The key to this structure is that the bases form specific base pairs(bp)held together by hydrogen bonds(Section 1.3):ade- nine pairs with thymine(A-T)and guanine pairs with cytosine(G-C),as shown in Figure 1.6.Hydrogen bonds are much weaker than covalent bonds such as the carbon-carbon or carbon-nitrogen bonds that define the struc- tures of the bases themselves.Such weak bonds are crucial to biochemical systems;they are weak enough to be reversibly broken in biochemical pro- cesses,yet they are strong enough,when many form simultaneously,to help stabilize specific structures such as the double helix. DNA structure explains heredity and the storage of information The structure proposed by Watson and Crick has two properties of central importance to the role of DNA as the hereditary material.First,the struc- ture is compatible with any sequence of bases.The base pairs have essen- tially the same shape(see Figure 1.6)and thus fit equally well into the center of the double-helical structure of any sequence.Without any constraints, the sequence of bases along a DNA strand can act as an efficient means of storing information.Indeed,the sequence of bases along DNA strands is how genetic information is stored.The DNA sequence determines the sequences of the ribonucleic acid(RNA)and protein molecules that carry out most of the activities within cells. Second,because of base-pairing,the sequence of bases along one strand completely determines the sequence along the other strand.As Watson and Crick so coyly wrote:"It has not escaped our notice that the specific pairing H H CHs H N-H------N N-H-----. H Adenine (A) Thymine(T) Guanine(G) Cytosine(C) Figure 1.6 Watson-Crick base pairs.Adenine pairs with thymine(A-T),and guanine with cytosine (G-C).The dashed green lines represent hydrogen bonds
5 1.2 DNA: Form and Function Two single strands of DNA combine to form a double helix Most DNA molecules consist of not one but two strands (Figure 1.5). In 1953, James Watson and Francis Crick deduced the arrangement of these strands and proposed a three-dimensional structure for DNA molecules. This structure is a double helix composed of two intertwined strands arranged such that the sugar–phosphate backbone lies on the outside and the bases on the inside. The key to this structure is that the bases form specific base pairs (bp) held together by hydrogen bonds (Section 1.3): adenine pairs with thymine (A–T) and guanine pairs with cytosine (G–C), as shown in Figure 1.6. Hydrogen bonds are much weaker than covalent bonds such as the carbon–carbon or carbon–nitrogen bonds that define the structures of the bases themselves. Such weak bonds are crucial to biochemical systems; they are weak enough to be reversibly broken in biochemical processes, yet they are strong enough, when many form simultaneously, to help stabilize specific structures such as the double helix. DNA structure explains heredity and the storage of information The structure proposed by Watson and Crick has two properties of central importance to the role of DNA as the hereditary material. First, the structure is compatible with any sequence of bases. The base pairs have essentially the same shape (see Figure 1.6) and thus fit equally well into the center of the double-helical structure of any sequence. Without any constraints, the sequence of bases along a DNA strand can act as an efficient means of storing information. Indeed, the sequence of bases along DNA strands is how genetic information is stored. The DNA sequence determines the sequences of the ribonucleic acid (RNA) and protein molecules that carry out most of the activities within cells. Second, because of base-pairing, the sequence of bases along one strand completely determines the sequence along the other strand. As Watson and Crick so coyly wrote: “It has not escaped our notice that the specific pairing Figure 1.5 The double helix. The double-helical structure of DNA proposed by Watson and Crick. The sugar–phosphate backbones of the two chains are shown in red and blue, and the bases are shown in green, purple, orange, and yellow. The two strands are antiparallel, running in opposite directions with respect to the axis of the double helix, as indicated by the arrows. N N N N N H H N N O O H CH3 Adenine (A) Thymine (T) N N N N O N H H H N N O N H H Guanine (G) Cytosine (C) Figure 1.6 Watson–Crick base pairs. Adenine pairs with thymine (A–T), and guanine with cytosine (G–C). The dashed green lines represent hydrogen bonds
6 we have postulated immediately suggests a possible copying mechanism for CHAPTER 1 Biochemistry: the genetic material."Thus,if the DNA double helix is separated into two An Evolving Science single strands,each strand can act as a template for the generation of its partner strand through specific base-pair formation(Figure 1.7).The three- dimensional structure of DNA beautifully illustrates the close connection between molecular form and function. Newly synthesized 1.3 Concepts from Chemistry Explain the Properties of strands Biological Molecules We have seen how a chemical insight,into the hydrogen-bonding capabili- ties of the bases of DNA,led to a deep understanding of a fundamental biological process.To lay the groundwork for the rest of the book,we begin our study of biochemistry by examining selected concepts from chemistry and showing how these concepts apply to biological systems.The concepts Figure 1.7 DNA replication.If a DNA molecule is separated into two strands,each include the types of chemical bonds;the structure of water,the solvent in strand can act as the template for the which most biochemical processes take place;the First and Second Laws of generation of its partner strand. Thermodynamics;and the principles of acid-base chemistry.We will use these concepts to examine an archetypical biochemical process-namely, the formation of a DNA double helix from its two component strands.The process is but one of many examples that could have been chosen to illus- trate these topics.Keep in mind that,although the specific discussion is about DNA and double-helix formation,the concepts considered are quite general and will apply to many other classes of molecules and processes that will be discussed in the remainder of the book. The double helix can form from its component strands The discovery that DNA from natural sources exists in a double-helical form with Watson-Crick base pairs suggested,but did not prove,that such double helices would form spontaneously outside biological systems Suppose that two short strands of DNA were chemically synthesized to have complementary sequences so that they could,in principle,form a double helix with Watson-Crick base pairs.Two such sequences are CGATTAAT and ATTAATCG.The structures of these molecules in solution can be examined by a variety of techniques.In isolation,each sequence exists almost exclusively as a single-stranded molecule.However, when the two sequences are mixed,a double helix with Watson-Crick base pairs does form(Figure 1.8).This reaction proceeds nearly to completion. Figure 1.8 Formation of a double helix. When two DNA strands with appropriate, complementary sequences are mixed,they spontaneously assemble to form a double helix. What forces cause the two strands of DNA to bind to each other?To analyze this binding reaction,we must consider several factors:the types of interactions and bonds in biochemical systems and the energetic favor- ability of the reaction.We must also consider the influence of the solution conditions-in particular,the consequences of acid-base reactions
6 CHAPTER 1 Biochemistry: An Evolving Science we have postulated immediately suggests a possible copying mechanism for the genetic material.” Thus, if the DNA double helix is separated into two single strands, each strand can act as a template for the generation of its partner strand through specific base-pair formation (Figure 1.7). The threedimensional structure of DNA beautifully illustrates the close connection between molecular form and function. 1.3 Concepts from Chemistry Explain the Properties of Biological Molecules We have seen how a chemical insight, into the hydrogen-bonding capabilities of the bases of DNA, led to a deep understanding of a fundamental biological process. To lay the groundwork for the rest of the book, we begin our study of biochemistry by examining selected concepts from chemistry and showing how these concepts apply to biological systems. The concepts include the types of chemical bonds; the structure of water, the solvent in which most biochemical processes take place; the First and Second Laws of Thermodynamics; and the principles of acid–base chemistry. We will use these concepts to examine an archetypical biochemical process—namely, the formation of a DNA double helix from its two component strands. The process is but one of many examples that could have been chosen to illustrate these topics. Keep in mind that, although the specific discussion is about DNA and double-helix formation, the concepts considered are quite general and will apply to many other classes of molecules and processes that will be discussed in the remainder of the book. The double helix can form from its component strands The discovery that DNA from natural sources exists in a double-helical form with Watson–Crick base pairs suggested, but did not prove, that such double helices would form spontaneously outside biological systems. Suppose that two short strands of DNA were chemically synthesized to have complementary sequences so that they could, in principle, form a double helix with Watson–Crick base pairs. Two such sequences are CGATTAAT and ATTAATCG. The structures of these molecules in solution can be examined by a variety of techniques. In isolation, each sequence exists almost exclusively as a single-stranded molecule. However, when the two sequences are mixed, a double helix with Watson–Crick base pairs does form (Figure 1.8). This reaction proceeds nearly to completion. C G T A T A A T C C G C A T G C G G C G T A C G Newly synthesized strands Figure 1.7 DNA replication. If a DNA molecule is separated into two strands, each strand can act as the template for the generation of its partner strand. G T T T A A A C G C A A A T T T G T T T A A A G C A A A T T T C Figure 1.8 Formation of a double helix. When two DNA strands with appropriate, complementary sequences are mixed, they spontaneously assemble to form a double helix. What forces cause the two strands of DNA to bind to each other? To analyze this binding reaction, we must consider several factors: the types of interactions and bonds in biochemical systems and the energetic favorability of the reaction. We must also consider the influence of the solution conditions—in particular, the consequences of acid–base reactions
Covalent and noncovalent bonds are important for the structure and 1 stability of biological molecules 1.3 Chemical Concepts Atoms interact with one another through chemical bonds.These bonds include the covalent bonds that define the structure of molecules as well as a variety of noncovalent bonds that are of great importance to biochemistry. Covalent bonds.The strongest bonds are covalent bonds,such as the Distance and energy units bonds that hold the atoms together within the individual bases shown on InterZatomic distances and bond lengths page 4.A covalent bond is formed by the sharing of a pair of electrons are usually measured in angstrom (A)units: between adjacent atoms.A typical carbon-carbon(C-C)covalent bond has a bond length of 1.54 A and bond energy of 355 kJ mol(85 kcal mol). 1A=10-10m=10-8cm=0.1nm Because covalent bonds are so strong,considerable energy must be expended Several energy units are in common to break them.More than one electron pair can be shared between two use.One joule (is the amount of energy atoms to form a multiple covalent bond.For example,three of the bases in required to move 1 meter against a force of 1 newton.A kilojoule (kJ)is 1000 joules Figure 1.6 include carbon-oxygen(C=O)double bonds.These bonds are One calorie is the amount of energy even stronger than C-C single bonds,with energies near 730 kJ mol-1 required to raise the temperature of 1 gram (175 kcal mol)and are somewhat shorter. of water 1 degree Celsius.A kilocalorie (kcal) For some molecules,more than one pattern of covalent bonding can be is 1000 calories.One joule is equal to written.For example,adenine can be written in two equivalent ways called 0.239cal. resonance structures NH2 NH> These adenine structures depict alternative arrangements of single and double bonds that are possible within the same structural framework. Resonance structures are shown connected by a double-headed arrow. Adenine's true structure is a composite of its two resonance structures.The composite structure is manifested in the bond lengths such as that for the bond joining carbon atoms C-4 and C-5.The observed bond length of 1.40 A is between that expected for a C-C single bond(1.54 A)and a C=C double bond(1.34 A).A molecule that can be written as several resonance structures of approximately equal energies has greater stability than does a molecule without multiple resonance structures. Noncovalent bonds.Noncovalent bonds are weaker than covalent bonds but are crucial for biochemical processes such as the formation of a double helix.Four fundamental noncovalent bond types areelectrostatic interactions, hydrogen bonds,van der Waals interactions,and hydrophobic interactions.They differ in geometry,strength,and specificity.Furthermore,these bonds are affected in vastly different ways by the presence of water.Let us consider the characteristics of each type: 1.Electrostatic Interactions.A charged group on one molecule can attract an oppositely charged group on another molecule.The energy of an electro- static interaction is given by Coulomb's law: @ 2 E kq1q2/Dr where E is the energy,qi and q2 are the charges on the two atoms(in units of the electronic charge),ris the distance between the two atoms(in angstroms), D is the dielectric constant(which accounts for the effects of the intervening
7 1.3 Chemical Concepts Covalent and noncovalent bonds are important for the structure and stability of biological molecules Atoms interact with one another through chemical bonds. These bonds include the covalent bonds that define the structure of molecules as well as a variety of noncovalent bonds that are of great importance to biochemistry. Covalent bonds. The strongest bonds are covalent bonds, such as the bonds that hold the atoms together within the individual bases shown on page 4. A covalent bond is formed by the sharing of a pair of electrons between adjacent atoms. A typical carbon–carbon (COC) covalent bond has a bond length of 1.54 Å and bond energy of 355 kJ mol21 (85 kcal mol21 ). Because covalent bonds are so strong, considerable energy must be expended to break them. More than one electron pair can be shared between two atoms to form a multiple covalent bond. For example, three of the bases in Figure 1.6 include carbon–oxygen (CPO) double bonds. These bonds are even stronger than COC single bonds, with energies near 730 kJ mol21 (175 kcal mol21 ) and are somewhat shorter. For some molecules, more than one pattern of covalent bonding can be written. For example, adenine can be written in two equivalent ways called resonance structures. N N N N H H NH2 5 4 N N N N H H NH2 5 4 These adenine structures depict alternative arrangements of single and double bonds that are possible within the same structural framework. Resonance structures are shown connected by a double-headed arrow. Adenine’s true structure is a composite of its two resonance structures. The composite structure is manifested in the bond lengths such as that for the bond joining carbon atoms C-4 and C-5. The observed bond length of 1.40 Å is between that expected for a COC single bond (1.54 Å) and a CPC double bond (1.34 Å). A molecule that can be written as several resonance structures of approximately equal energies has greater stability than does a molecule without multiple resonance structures. Noncovalent bonds. Noncovalent bonds are weaker than covalent bonds but are crucial for biochemical processes such as the formation of a double helix. Four fundamental noncovalent bond types are electrostatic interactions, hydrogen bonds, van der Waals interactions, and hydrophobic interactions. They differ in geometry, strength, and specificity. Furthermore, these bonds are affected in vastly different ways by the presence of water. Let us consider the characteristics of each type: 1. Electrostatic Interactions. A charged group on one molecule can attract an oppositely charged group on another molecule. The energy of an electrostatic interaction is given by Coulomb’s law: E 5 kq1q2yDr where E is the energy, q1 and q2 are the charges on the two atoms (in units of the electronic charge), r is the distance between the two atoms (in angstroms), D is the dielectric constant (which accounts for the effects of the intervening InterZatomic distances and bond lengths are usually measured in angstrom (Å) units: 1 Å 5 10210 m 5 1028 cm 5 0.1 nm Several energy units are in common use. One joule (J) is the amount of energy required to move 1 meter against a force of 1 newton. A kilojoule (kJ) is 1000 joules. One calorie is the amount of energy required to raise the temperature of 1 gram of water 1 degree Celsius. A kilocalorie (kcal) is 1000 calories. One joule is equal to 0.239 cal. Distance and energy units q1 q2 r
medium),and k is a proportionality constant(k=1389,for energies in units CHAPTER 1 Biochemistry: of kilojoules per mole,or 332 for energies in kilocalories per mole). An Evolving Science By convention,an attractive interaction has a negative energy.The elec- trostatic interaction between two ions bearing single opposite charges sepa- rated by 3 A in water(which has a dielectric constant of 80)has an energy of 5.8 kJ mol(-1.4 kcal mol).Note how important the dielectric constant of the medium is.For the same ions separated by 3 A in a nonpolar solvent such as hexane(which has a dielectric constant of 2),the energy of this inter- action is-232 kJ mol(-55 kcal mol). Hydrogen- Hydrogen- 2.Hydrogen Bonds.These interactions are fundamentally electrostatic bond donor bond acceptor interactions.Hydrogen bonds are responsible for specific base-pair for- N一… mation in the DNA double helix.The hydrogen atom in a hydrogen bond N—H----0 is partially shared by two electronegative atoms such as nitrogen or oxy- gen.The hydrogen-bond donor is the group that includes both the atom to O—H---N which the hydrogen atom is more tightly linked and the hydrogen atom 0-H---0 itself,whereas the hydrogen-bond acceptor is the atom less tightly linked to the hydrogen atom(Figure 1.9).The electronegative atom to which the Figure 1.9 Hydrogen bonds.Hydrogen hydrogen atom is covalently bonded pulls electron density away from the bonds are depicted by dashed green lines.The positions of the partial charges hydrogen atom,which thus develops a partial positive charge(5).Thus, and are shown. the hydrogen atom can interact with an atom having a partial negative charge(8 )through an electrostatic interaction. Hydrogen bonds are much weaker than covalent bonds.They have ener- gies ranging from 4 to 20 kJ mol(from 1 to 5 kcal mol).Hydrogen Hydrogen- Hydrogen-bond bond donor bonds are also somewhat longer than covalent bonds;their bond lengths acceptor (measured from the hydrogen atom)range from 1.5 A to 2.6 A;hence,a dis- ￥0.9A2.0A N-H---...0 tance ranging from 2.4 A to 3.5 A separates the two nonhydrogen atoms in a hydrogen bond.The strongest hydrogen bonds have a tendency to be 180 approximately straight,such that the hydrogen-bond donor,the hydrogen atom,and the hydrogen-bond acceptor lie along a straight line.Hydrogen- bonding interactions are responsible for many of the properties of water that make it such a special solvent,as will be described shortly. 3.van der Waals Interactions.The basis of a van der Waals interaction is that the distribution of electronic charge around an atom fluctuates with van der Waals time.At any instant,the charge distribution is not perfectly symmetric. contact distance This transient asymmetry in the electronic charge about an atom acts Distance through electrostatic interactions to induce a complementary asymmetry in the electron distribution within its neighboring atoms.The atom and its neighbors then attract one another.This attraction increases as two atoms come closer to each other,until they are separated by the van der Waals contact distance(Figure 1.10).At distances shorter than the van der Waals contact distance,very strong repulsive forces become dominant because the outer electron clouds of the two atoms overlap. Figure 1.10 Energy of a van der Waals interaction as two atoms approach each Energies associated with van der Waals interactions are quite small; other.The energy is most favorable at the typical interactions contribute from 2 to 4 kJ mol(from 0.5 to 1 kcal van der Waals contact distance.Owing to mol)per atom pair.When the surfaces of two large molecules come electron-electron repulsion,the energy rises together,however,a large number of atoms are in van der Waals contact, rapidly as the distance between the atoms and the net effect,summed over many atom pairs,can be substantial. becomes shorter than the contact distance. Properties of water.Water is the solvent in which most biochemical reac- tions take place,and its properties are essential to the formation of macro- molecular structures and the progress of chemical reactions.Two properties Electric of water are especially relevant: dipole 1.Water is a polar molecule.The water molecule is bent,not linear,and so the distribution of charge is asymmetric.The oxygen nucleus draws elec-
8 CHAPTER 1 Biochemistry: An Evolving Science medium), and k is a proportionality constant (k 5 1389, for energies in units of kilojoules per mole, or 332 for energies in kilocalories per mole). By convention, an attractive interaction has a negative energy. The electrostatic interaction between two ions bearing single opposite charges separated by 3 Å in water (which has a dielectric constant of 80) has an energy of 5.8 kJ mol21 (21.4 kcal mol21 ). Note how important the dielectric constant of the medium is. For the same ions separated by 3 Å in a nonpolar solvent such as hexane (which has a dielectric constant of 2), the energy of this interaction is 2232 kJ mol21 (255 kcal mol21 ). 2. Hydrogen Bonds. These interactions are fundamentally electrostatic interactions. Hydrogen bonds are responsible for specific base-pair formation in the DNA double helix. The hydrogen atom in a hydrogen bond is partially shared by two electronegative atoms such as nitrogen or oxygen. The hydrogen-bond donor is the group that includes both the atom to which the hydrogen atom is more tightly linked and the hydrogen atom itself, whereas the hydrogen-bond acceptor is the atom less tightly linked to the hydrogen atom (Figure 1.9). The electronegative atom to which the hydrogen atom is covalently bonded pulls electron density away from the hydrogen atom, which thus develops a partial positive charge (d1). Thus, the hydrogen atom can interact with an atom having a partial negative charge (d2) through an electrostatic interaction. Hydrogen bonds are much weaker than covalent bonds. They have energies ranging from 4 to 20 kJ mol21 (from 1 to 5 kcal mol21 ). Hydrogen bonds are also somewhat longer than covalent bonds; their bond lengths (measured from the hydrogen atom) range from 1.5 Å to 2.6 Å; hence, a distance ranging from 2.4 Å to 3.5 Å separates the two nonhydrogen atoms in a hydrogen bond. The strongest hydrogen bonds have a tendency to be approximately straight, such that the hydrogen-bond donor, the hydrogen atom, and the hydrogen-bond acceptor lie along a straight line. Hydrogenbonding interactions are responsible for many of the properties of water that make it such a special solvent, as will be described shortly. 3. van der Waals Interactions. The basis of a van der Waals interaction is that the distribution of electronic charge around an atom fluctuates with time. At any instant, the charge distribution is not perfectly symmetric. This transient asymmetry in the electronic charge about an atom acts through electrostatic interactions to induce a complementary asymmetry in the electron distribution within its neighboring atoms. The atom and its neighbors then attract one another. This attraction increases as two atoms come closer to each other, until they are separated by the van der Waals contact distance (Figure 1.10). At distances shorter than the van der Waals contact distance, very strong repulsive forces become dominant because the outer electron clouds of the two atoms overlap. Energies associated with van der Waals interactions are quite small; typical interactions contribute from 2 to 4 kJ mol21 (from 0.5 to 1 kcal mol21 ) per atom pair. When the surfaces of two large molecules come together, however, a large number of atoms are in van der Waals contact, and the net effect, summed over many atom pairs, can be substantial. Properties of water. Water is the solvent in which most biochemical reactions take place, and its properties are essential to the formation of macromolecular structures and the progress of chemical reactions. Two properties of water are especially relevant: 1. Water is a polar molecule. The water molecule is bent, not linear, and so the distribution of charge is asymmetric. The oxygen nucleus draws elecN H N N H O O H N O H O Hydrogenbond donor Hydrogenbond acceptor + − − Figure 1.9 Hydrogen bonds. Hydrogen bonds are depicted by dashed green lines. The positions of the partial charges (d1 and d2) are shown. N H O 0.9 Å 2.0 Å 180° Hydrogenbond donor Hydrogen-bond acceptor Energy Attraction Repulsion 0 van der Waals contact distance Distance Figure 1.10 Energy of a van der Waals interaction as two atoms approach each other. The energy is most favorable at the van der Waals contact distance. Owing to electron–electron repulsion, the energy rises rapidly as the distance between the atoms becomes shorter than the contact distance. O H H + – Electric dipole
trons away from the two hydrogen nuclei,which leaves the region around 9 each hydrogen atom with a net positive charge.The water molecule is thus 1.3 Chemical Concepts an electrically polar structure. 2.Water is highly cohesive.Water molecules interact strongly with one another through hydrogen bonds.These interactions are apparent in the structure of ice(Figure 1.11).Networks of hydrogen bonds hold the struc- ture together;similar interactions link molecules in liquid water and account for the cohesion of liquid water,although,in the liquid state,approximately one-fourth of the hydrogen bonds present in ice are broken.The polar nature of water is responsible for its high dielectric constant of 80.Molecules in aqueous solution interact with water molecules through the formation of hydrogen bonds and through ionic interactions.These interactions make water a versatile solvent,able to readily dissolve many species,especially polar and charged compounds that can participate in these interactions. Figure 1.11 Structure of ice.Hydrogen bonds (shown as dashed green lines)are formed between water molecules to produce a highly ordered and open structure. The hydrophobic effect.A final fundamental interaction called the hydro- phobic effect is a manifestation of the properties of water.Some molecules (termed nonpolar molecules)cannot participate in hydrogen bonding or ionic interactions.The interactions of nonpolar molecules with water molecules are not as favorable as are interactions between the water molecules them- selves.The water molecules in contact with these nonpolar molecules form 'cages"around them,becoming more well ordered than water molecules free in solution.However,when two such nonpolar molecules come together,some of the water molecules are released,allowing them to interact freely with bulk water(Figure 1.12).The release of water from such cages is favorable for reasons to be considered shortly.The result is that nonpolar ap 9 p Nonpolar molecule Nonpolar d molecule Nonpolar Figure 1.12 The hydrophobic effect.The molecule aggregation of nonpolar groups in water leads 60 db. to the release of water molecules.initially Nonpolar molecule interacting with the nonpolar surface,into bulk 6 water.The release of water molecules into solution makes the aggregation of nonpolar groups favorable
trons away from the two hydrogen nuclei, which leaves the region around each hydrogen atom with a net positive charge. The water molecule is thus an electrically polar structure. 2. Water is highly cohesive. Water molecules interact strongly with one another through hydrogen bonds. These interactions are apparent in the structure of ice (Figure 1.11). Networks of hydrogen bonds hold the structure together; similar interactions link molecules in liquid water and account for the cohesion of liquid water, although, in the liquid state, approximately one-fourth of the hydrogen bonds present in ice are broken. The polar nature of water is responsible for its high dielectric constant of 80. Molecules in aqueous solution interact with water molecules through the formation of hydrogen bonds and through ionic interactions. These interactions make water a versatile solvent, able to readily dissolve many species, especially polar and charged compounds that can participate in these interactions. The hydrophobic effect. A final fundamental interaction called the hydrophobic effect is a manifestation of the properties of water. Some molecules (termed nonpolar molecules) cannot participate in hydrogen bonding or ionic interactions. The interactions of nonpolar molecules with water molecules are not as favorable as are interactions between the water molecules themselves. The water molecules in contact with these nonpolar molecules form “cages” around them, becoming more well ordered than water molecules free in solution. However, when two such nonpolar molecules come together, some of the water molecules are released, allowing them to interact freely with bulk water (Figure 1.12). The release of water from such cages is favorable for reasons to be considered shortly. The result is that nonpolar 9 1.3 Chemical Concepts Figure 1.11 Structure of ice. Hydrogen bonds (shown as dashed green lines) are formed between water molecules to produce a highly ordered and open structure. Nonpolar molecule Nonpolar molecule Nonpolar molecule Nonpolar molecule Figure 1.12 The hydrophobic effect. The aggregation of nonpolar groups in water leads to the release of water molecules, initially interacting with the nonpolar surface, into bulk water. The release of water molecules into solution makes the aggregation of nonpolar groups favorable